1. Field of the Invention
This invention generally relates to saturation detection circuits, in embodiments for substantially lossless detection of saturation of power switches in power integrated circuits.
2. Description of the Related Art
There are many types of power semiconductor devices which may be used for power switching including, for example, power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and Insulated Gate Bipolar Transistors (IGBTs). Broadly speaking when we refer to power devices we are generally referring to devices which are suitable for operating with a voltage of greater than 100 volts and of powers greater than 1 watt. Such power semiconductor devices may be incorporated into hybrid circuits and, more particularly, into power integrated circuits. In this latter case the power device often operates alongside low voltage devices performing a driver and/or control function. The low voltage circuitry is typically constructed using CMOS (Complementary Metal Oxide Semiconductor), bipolar or BiCMOS technology; the high voltage circuitry generally employs junction-isolation (JI) or semiconductor-on-insulator (SOI) technology.
A particularly advantageous form of construction for a power semiconductor device is described in the applicant's patents WO 02/25700, U.S. Pat. No. 6,703,684 and equivalents of an application derived from these patents, all of which are hereby incorporated by reference in their entirety. Broadly speaking these describe a power semiconductor device including a drift region in a membrane spaced away from the semiconductor substrate.
One method of current sensing in a power integrated circuit employs an on-chip or off-chip current sense resistor coupled to a comparator/reference circuit, but this technique is not able to protect a power switch such as an LIGBT (Lateral IGBT) or MOSFET from going into saturation. Furthermore power losses in the current sense resistor can be significant at higher powers. Generally the sense resistor also needs to be outside the power integrated circuit (IC). Thus although measuring the current through a current shunt resistor in the drain/emitter leg of a power device is straightforward to implement, it has a number of drawbacks.
Another technique is to employ a small parallel mirror switch with a current sense resistor in its source/emitter leg. This provides significantly reduced power consumption and enables the current sense finger to be monolithically integrated but still does not allow saturation to be detected. The data sheet of the Fairchild semiconductor FSDM311 power switch shows a circuit which appears to use this type of approach.
A further approach, used by Power Integrations, Inc. employs on-chip current sensing in which a lateral power MOSFET is provided with a low voltage tap in its drift region. This allows the on-state voltage of the device to be measured which (because of the on-state drain-source resistance of the MOSFET) is proportional to the current through the power switch.
Further background prior art can be found in: U.S. Pat. No. 5,631,494; US2004/0227539; EP0 516 848A; U.S. Pat. No. 5,656,968; and U.S. Pat. No. 5,737,169.
When a power semiconductor device goes into saturation its on-state voltage increases dramatically with very small increases in current. The power losses associated with this saturation can seriously damage or destroy the power device, especially in power integrated circuits where the power devices are generally less robust than discrete devices, and are often operated at their maximum ratings.
There is therefore a need for improved current sensing techniques, in particular improved techniques to enable saturation of a power switching device to be detected.